1. Field of the Invention
The present invention relates in general to systems which utilize light scattering principles to detect and count undesirable particles in fluids, referred to in the art as light scattering particle counters, and more particularly to high power, low noise intracavity laser particle counters.
2. Statement of the Problem
The history of the semiconductor industry has shown a consistent path of steadily decreasing line widths. The semiconductor industry roadmap for the future shows this trend continuing unabated. Smaller semiconductor line widths mean smaller critical defect sizes, which in turn require detection of smaller particles, for effective contamination monitoring of clean room air. Accordingly, the semiconductor roadmap continually pushes researchers to develop ever more sensitive OPCs (optical particle counters), to measure ever smaller particle sizes. To achieve a statistically valid sample in a reasonable amount of time, when operating in a very clean environment, high performance OPCs should also have high sample rates (volume of air sampled per unit time).
An existing particle detection system is described in U.S. Pat. No. 5,889,589 issued Mar. 30, 1999 to Jon C. Sandberg (the '589 patent), which patent is hereby incorporated herein by reference. The OPC of the '589 patent measures scattered laser radiation from particles which pass through a sample volume. The magnitude of the scattered laser radiation is proportional to particle size, and each particle generates a single optical pulse. This allows the particle counter to detect particles which pass through the sample volume. For particles much smaller than the laser wavelength (the “Rayleigh range”), the magnitude of scattered laser radiation is proportional to the sixth power of the particle diameter. Hence, it quickly becomes very difficult to measure smaller and smaller particles.
To take advantage of the high intracavity power of the solid-state laser, the sample air is directed through the laser's active cavity by an inlet jet, placed proximate to the laser beam. Sample air is drawn through the sample volume by applying a vacuum source to the outlet jet.
The laser medium is optically pumped by an optical pump source whose output is generally coupled through a focusing lens system. The laser medium element can be one of a family of crystals such as Nd:YAG, Nd:YLF, Nd:YALO, Nd:YVO4. By using a lens to focus the diode laser pump radiation to a small waist within the solid-state laser crystal, aperture control is obtained through gain-aperturing. This design leads to a single transverse mode and high intracavity power.
The '589 patent identifies several benefits of its gain-aperturing system over the prior art including enabling operation with weak dependence on the shape, size, and alignment of the pumped volume, reducing flow induced laser noise, and allowing high power operation. See the '589 patent, column 6, lines 11-27. The '589 patent also identifies problems associated with combining physical aperturing with, gain aperturing. After describing an embodiment in which an aperture having a diameter of about one millimeter is added to a gain-apertured system, the '589 patent asserts that “the presence of the physical, aperture adversely affects intracavity power and relative noise as flow rate is increased.” See the '589 patent, column 7, lines 6-8. Accordingly, the '589 patent specifically teaches away from combining physical aperturing with gain aperturing.
To accurately detect very small particles, such as 0.065 micrometer (μm) or still smaller particles, in a fluid flowing at a rate greater than or equal to 1.0 Cubic Feet per Minute (CFM), at an efficiency level of 30% or higher, it is highly desirable to provide a laser system having low noise as well as high power operation. The particle counter of the '589 patent experiences higher than desired noise levels, characterized by bursts of relatively large amplitude noise. Detector thresholds generally have to be set high enough to reject the worst case noise, so as to reject false counts. However, these elevated thresholds generally inhibited the effective detection of very small particles with a reasonable counting efficiency, at fluid flow rates greater than or equal to 1.0 CFM.
Accordingly, there is a need in the art to preserve high power laser operation while reducing the noise level of such lasers so as to permit effective detection of small particles at a desired counting efficiency at fluid flow rates equal to or less than 1.0 CFM.